Fuel cell sub-assembly and method of making it

ABSTRACT

A sub-assembly for an electrochemical stack, such as a PEM fuel cell stack, has a bipolar plate with sealing material extending from its upper face, around the edge of the bipolar plate, and onto its lower face. The bipolar plate is preferably a combination of an anode plate and a cathode plate defining an internal coolant flow field and bonded together by sealing material which also provides a seal around the coolant flow field. All of the sealing material in the sub-assembly may be one contiguous mass. To make the sub-assembly, anode and cathode plates are loaded into a mold. Liquid sealing material is injected into the mold and fills a gap between the edge of the plates, and portions of the outer faces of the plates, and the mold. In a stack, sub-assemblies are separated by MEAs which at least partially overlap the sealing material on their faces.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 61/885,652 filed on Oct. 2, 2013 which is incorporatedherein by reference.

FIELD

This specification relates to electrochemical cells, such as fuel cells,and in particular to a sub-assembly of plates and seals for use inmaking in a cell stack, and to methods of making the sub-assembly.

BACKGROUND

A proton exchange membrane (PEM) fuel cell (PEMFC), alternatively calleda polymer electrolyte membrane fuel cell, typically comprises an anodeplate and a cathode plate separated by a membrane electrode assembly(MEA), typically with a gas diffusion layer (GDL) between each side ofthe MEA and its adjacent plate. The surfaces of the anode plate andcathode plate that face the MEA are shaped to provide a flow field forthe reactant gasses, typically hydrogen and air. A PEM fuel cell stackcomprises an assembly of fuel cells clamped between and end plates, endplate insulator and current collector at each end of the stack. In thestack, the anode plate and cathode plate of adjacent fuel cells areelectrically connected and may be provided by a bipolar. The bipolarplate may be a unitary structure or an anode plate and cathode platebonded together. Coolant flow fields may be provided between adjacentfuel cells, either between every pair of successive fuel cells or atsome lesser interval, for example after every second to fifth fuel cell.The coolant flow fields may be provided within a bipolar plate, betweenabutting anode and cathode plates, or in a separate plate. Typically,there are also various holes through the thickness of the plates. Theseholes collectively define conduits through the stack (perpendicular tothe plates) to transport reactants, reaction products, or coolant to orfrom the individual fuel cells. Seals are required between each flowfield and the adjacent MEA. Seals are also required around the holes inthe plates, and between the holes and their associated flow fields.Seals may also be required around coolant flow fields. Optionally, sealsmay also electrically insulate the anode plate and cathode plate of afuel cell, or between adjacent bipolar plates. Due to the large numberof seals and plates in a fuel cell stack, methods of making andassembling these components are constantly in need of alternatives toprovide improvements or to be suited to selected manufacturingtechniques and materials.

In U.S. Pat. No. 6,599,653, anode and cathode plates are molded fromplastic composites that include graphite. The anode and cathode platesare made into a sub-assembly called a fuel cell unit. Each fuel cellunit also includes an insulation layer on the bottom of the anode plate,a bead of sealant between the anode plate and the cathode plate, andanother bead of sealant on the top of the cathode plate.

The anode and cathode plate have aligned gates to facilitate the flow ofa curable liquid silicone through the plates and grooves to receive thebeads of sealant. A fuel cell unit is made by placing an anode plate andcathode plate on the floor of a mold with the anode plate spaced fromthe floor of the mold. Liquid silicone is then forced through the gatesand into the space between the anode plate and the floor of the mold.When the silicone cures, the insulation layer and the two beads ofsealant are formed as a unitary, contiguous mass. This mass bonds theanode plate and cathode plate together and provides an insulation layerand seal on opposed sides of the bonded plates.

U.S. Pat. No. 7,210,220 describes a sealing technique for fuel cells andother electrochemical cells. To provide a seal, a groove network isprovided through various elements of a fuel cell assembly. One fuel cellassembly includes anode and cathode plates, MEAs and GDLs for severalfuel cells, all clamped together between end plates, end plateinsulators and current collectors. Insulating material is providedbetween the anode and cathode plates of each fuel cell to prevent shortsacross the fuel cells. The insulation may be provided as part of anadjacent MEA (for example as a non-conductive flange bonded to the MEA),by a GDL which extends to the edge of the plate, or by using plates thatare made non-conductive or covered with an insulator in these areas. Asource of seal material is then connected to an external filling portand injected into the groove network. When the sealing material cures,it forms a “seal in place” that bonds and seals the fuel cell assemblyelements. In an alternative embodiment, a Membrane Electrode Unit (MEU)is made which comprises 1 to 5 sealed in place fuel cells. At least oneof the outer faces of the MEU has an outer seal. This outer seal isadapted to seal to another MEU. Typically, an outer face of the MEU isadapted to form a cooling chamber with the other MEU. A fuel cell stackis produced by assembling any number of MEUs with end plates, end plateinsulators and current collectors.

SUMMARY OF THE INVENTION

The following summary is intended to introduce the reader to thedetailed description to follow and not to limit or define any claimedinvention.

A sub-assembly for an electrochemical stack described in thisspecification has a bipolar plate with flow fields on its upper andlower faces, and a sealing material bonded to the bipolar plate. Thesealing material extends from the upper face of the bipolar plate,around the edge of the bipolar plate, and onto the lower face of theplate. Preferably, the sealing material also forms a bead around theperiphery of one or both of the flow fields. Preferably, the sealingmaterial also forms beads around one or more holes for reactant,combustion product, or coolant flow through the bipolar plate.

The bipolar plate may be a unitary structure or, preferably, acombination of an anode plate and a cathode plate bonded together andhaving an internal coolant flow field. In this case, the anode plate andthe cathode plate may be bonded together by sealing material which alsoprovides a seal around the coolant flow field. One or both of the platespreferably has one or more gates through its thickness, or extendinginwards from its edge, to allow liquid sealing material to be injectedbetween the plates. Optionally, all of the sealing material in thesub-assembly may be one contiguous mass.

In a method of making a sub-assembly described in this specification, asingle anode plate and a single cathode plate are loaded into a mold ina liquid injection molding machine such that reactant flow fields on theplates face away from each other. A liquid sealing material, for exampleliquid silicone rubber, is injected into the mold and fills a gapbetween the edge of the plates, and portions of the outer faces of theplates, and the mold. The liquid sealing material may also flow throughvarious grooves or gates, or both, of the plates. Preferably, sealingmaterial extending around the edges of the plates, sealing materialbonding the anode and cathode plates together, and sealing materialsealing around a coolant flow field between the plates, are all appliedwhile the plates are in a single mold. Preferably, all of the sealingmaterial applied to the plates merges into a single mass.

An electrochemical cell stack, for example a PEM fuel cell stack,described in this specification has a plurality of sub-assemblies asdescribed above, or sub-assemblies made by the method described above.Within the stack, a GDL is located on the upper face of a lowersub-assembly, preferably within the sealing material on the upper faceof the lower sub-assembly. An MEA is located over the GDL and at leastpartially overlaps with the sealing material on the upper face of thelower sub-assembly. A second GDL is located over the MEA, preferablywithin the sealing material on the lower face of an upper sub-assembly.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic plan view of a face of a sub-assembly.

FIG. 2 is a schematic plan view of another face of the sub-assembly ofFIG. 1.

FIGS. 3 and 6 are schematic cross sections of portions of thesub-assembly of FIGS. 1 and 2.

FIGS. 4 and 5 are schematic cross sections of portions of alternativesub-assemblies.

DETAILED DESCRIPTION

FIGS. 1 and 2 show a sub-assembly 10 for an electrochemical cell, forexample a PEM fuel cell. The sub-assembly 10 has an anode plate 12 and acathode plate 14 located back to back. The anode plate 12 is visible inFIG. 1 and the cathode plate is visible in FIG. 2. The plates 12, 14have faces 30, which are visible in FIGS. 1 and 2, and edges 32, whichare shown as lines bordering the faces 30 in FIGS. 1 and 2. Either ofthe outer faces 30, visible in FIGS. 1 and 2, may be called an upper ora lower face 30 depending on the orientation of the sub-assembly 10.Inner faces 30 of the plates 12, 14 contact each other and are notvisible in FIGS. 1 and 2 but appear as lines in the other Figures. Theplates 12, 14 are made of a conductive material. For example, the plates12, 14 may be made of a metal such as stainless steel or, preferably, amolded composite of a plastic or other resin mixed with graphite.

The sub-assembly 10 also has one or more masses of cured sealingmaterial 16. The sealing material 16 may be made by curing any suitablecurable liquid such as liquid silicone rubber (LSR), a polysiloxaneelastomeric material as described in U.S. Pat. No. 7,210,220, anethylene acrylic polymer, an ethylene propylene terpolymer, an epoxyresin or a thermoplastic elastomer. The sub-assembly 10 does not includea gas diffusion layer (GDL) or membrane electrode assembly (MEA) andinstead consists essentially of the plates 12, 14 and cured sealingmaterial 16. Preferably, the sub-assembly 10 consists only of the plates12, 14 and sealing material 16.

Preferably, the plates 12, 14 are not bonded together other than by thesealing material 16. Optionally, the plates 12, 14 may be separatelybonded together, for example by an epoxy resin mixed with graphite andapplied to the inside face of one or both of the plates 12, 14. However,this requires an extra step, and additional manufacturing equipment andspace, all of which can be avoided by using the sealing material 16 tobond the plates 12, 14.

One or both of the plates 12, 14, for example the anode plate 12,preferably defines a coolant flow field 18 between the plates 12, 14.The outer face of the anode plate 12 also defines an anode flow field 20and the outer face of the cathode plate 14 defines a cathode flow field22. The flow fields 18, 20, 22 are typically more complex than what isshown in FIGS. 1 and 2.

Aligned openings 24 through the plates 12, 14 define parts of conduits.The openings 24 may be collected at the ends of the plates 12, 14 asshown, or provided in different locations. In the sub-assembly 10 shown,one opening 24 a is provided to supply a reactant, typically air, to thecathode flow field 22 and then to a second opening 24 b provided toremove excess air, or nitrogen, and water. A third opening 24 c isprovided to input another reactant, for example hydrogen, to the anodeflow field 20 and then to a fourth opening 24 d to remove excesshydrogen. A fifth opening 24 e is provided to supply a coolant, forexample water or water mixed with an anti-freezing agent, to the coolantflow field 18 and then out through a sixth opening 24 f. Optionally,more or less openings 24 may be used. For example, in an air cooled cellstack, the coolant flow field 18 is open at two opposed sides of theplates 12, 14 and openings 24 for coolant flow are not required.

The plates 12, 14 provide a bipolar plate with an internal coolant flowfield 18. To create a PEM fuel cell stack, two or more sub-assemblies 10are stacked together. A gas diffusion layer (GDL), a membrane electrodeassembly (MEA) and a second GDL are placed between successivesub-assemblies 10. The gas diffusion layers extend generally across theanode flow field 20 and cathode flow field 22, but preferably end withinsealing material 16 on the faces 30 of the plates 12, 14. The MEAextends across the anode flow field 20 and the cathode flow field 22,and at least overlaps with sealing material 16 on the faces 30 of theplates 12, 14. Optionally, the MEA may also extend from the reactantflow fields 20, 22 and overlap with sealing material surrounding one ormore openings 24 that define reactant conduits. In this way, thereactants are sealed on opposite sides of the MEA. Passages 34 in theplates 12, 14 between the openings 24 and the flow fields 18, 20, 22 areshown in a simplified form in FIGS. 1 and 2 but can also be provided inother configurations known in the art. For example, whereas FIGS. 1 and2 show a “back side feed” configuration in which passages 34 forreactants are provided on inner faces 30 of the plates, the passages 34may alternatively be located in the outer faces 30 of the plates 12, 14.

Sealing material 16 is applied to the plates 12, 14 in a liquid form andthen cured on, and preferably between, the plates 12, 14. The plates 12,14 are placed in a mold having recesses to define outer surfaces of thesealing material 16 on the outer faces 30 and edges 32 of the plates 12,14. This mold is placed in a liquid injection molding (LIM) press orother suitable molding machine. The liquid sealing material, preferablyliquid silicone rubber, is then injected into the mold and cured. Ventsare provided in the mold or the plates 12, 14 to allow air to escape asthe sealing material 16 is injected into the mold. The size, number andlocation of mold injection points and vents can be determined by methodsknown in the art of injection molding. Injected liquid sealing material16 flows quickly around the periphery of the plates in the injectionmold which advantageously reduces the number of injection points to themold that are required and can also reduce, or optionally eliminate, theneed for injection molding gates through the thickness of the plates 12,14.

When using composite molded plates 12, 14, some water (or anothercoolant fluid) may diffuse from the coolant flow field 18 through theplates 12, 14 themselves. Water (or vapor) that diffuses into thereactant flow fields 20, 22 is carried away with the flows of thereactants or reaction products and typically causes no harm. However,some water can also appear at the edges of the plates 12, 14. This watercan cause problems, such as shorting between adjacent fuel cells orinterference with balance of plant elements around a stack. For thisreason, the sealing material 16 preferably includes an edge sealingportion 26 that wraps around the edges 32 of the plates 12, 14. The edgesealing portion 26 also electrically isolates the edges 32 of the plates12, 14, which is useful even with metal plates 12, 14. The edge sealingportion 26 is preferably contiguous around the entire periphery of theplates 12, 14. The MEAs preferably do not extend to the edges 32 of theplates. In this way, the side of a complete stack is practicallyinsulated in that a solid conductor touching the outside of a stack isunlikely to cause a short.

The sealing material 16 preferably provides various bead portions 28.The bead portions 28 seal the reactants on either side of the MEA andmay also help seal between the openings 24 of adjacent sub-assemblies 10in a stack. The shape of the bead portions 28 is selected to produce asufficient pressure against the MEA when a stack is clamped together.Preferably, the bead portions are located over grooves 42 in the plates12, 14. The bead portions 28 may be provided on one or both sides of theplates 12, 14 around the peripheries of the reactant flow fields 20, 22.Preferably, the edge sealing portion 26 of the sealing material 16extends from a bead portion 28 on one outer face 30 of the sub-assembly10 to the bead portion 28 on the other outer face 30 of the sub-assembly10 to form one continuous mass of sealing material. The bead portions 28are made thicker than adjacent parts of the edge sealing portion 26 onthe outer face of a plate 12, 14. This is avoids needlessly increasingthe total force that would be required to provide sufficient compressionin the bead portions 28. It also helps allow the deformation of thesealing material 16 to be consistent as between bead portions 28 locatednear the edges 32 of the plates 12, 14 and bead portions 28 displacedfrom the edges 32 by openings 24. Optionally, additional bead portions28 may be located near at or the edges 32 of the plates 12, 14, beyondthe area that will be overlapped by the MEA, to better insulate theedges of the MEA from the sides of a stack.

FIG. 3 shows a portion of an alternative sub-assembly 10 a in crosssection. In this portion of the sub-assembly 10 a there is no opening 24and a coolant flow field 18 extends to near the edge 32 of the plates12, 14. The thickness of the plates 12, 14 is exaggerated in FIG. 3 (andin FIGS. 4 to 6) and each may be on the order of 1 mm.

Although it is possible for the edges 32 of the plates 12, 14 to form asingle plane as in FIGS. 1 and 2, the resulting edge sealing portion 26alone might not provide an adequate seal around an internal coolantfield 18. In FIG. 3, the edges 32 of the plates 12, 14 have a step 40 toprovide additional sealing material 16 near the inner faces 30 of theplates 12, 14. Preferably, the step 40 is provided around the entireperiphery of the plates 12, 14. Optionally, the step 40 could beprovided in only the anode plate 12 or only the cathode plate 14.Alternatively, the step 40 may have another profile rather than thegenerally rectangular notch shown.

FIG. 4 shows a portion of another alternative sub-assembly 10 b in crosssection. In this case, a key 44 is provided in the anode plate 12.Optionally, the key 44 could be provided in the cathode plate 14 or keys44 could be provided in both plates 12, 14. The key 44 again providesadditional sealing material near the inner faces of the plates 12, 14.In addition, the key 44 mechanically locks the edge sealing portion 26of the sealing material 16 to the edges 32 of the plates. For thispurpose, the key 44 is preferably provided around the entire peripheryof the plates 12, 14. The key 44 may have a profile other than theprofile shown.

FIG. 5 shows a portion of another alternative sub-assembly 10 c in crosssection. In this case, a groove 42 is located on the inner face 30 ofthe anode plate 12. Optionally, a groove 42 may be located on the innerface 30 of the cathode plate 12, or on the inner faces 30 of both plates12, 14. This groove 42 may be located directly below, or overlappingwith, a groove 42 on an outer face 30 of the same plate 12, 14. However,it is preferable for a groove 42 on an inner face 30 of a plate 12, 14to be located either inside (away from the edge 32) or outside (towardsthe edge 32) of a groove 42 on an outer face 30 of the same plate 12, 14to avoid having a very thin section in the plate 12, 14. Liquid sealingmaterial 16 may be fed to a groove 42 on an inner face 30 of a plate 12,14 through one or more gates 46. The gates 46 may, for example, passthrough the thickness of a plate 12, 14. Alternatively, or additionally,gates 46 may be provided in the form of channels molded into the innerface 30 of a plate 12, 14 and contiguous with the edge 32 of the plate12, 14.

FIG. 6 shows another portion of the alternative sub-assembly 10 a ofFIG. 3 in cross section. In this portion of the sub-assembly 10 a thereis an opening 24 between the coolant flow field 18 and the edges 32 ofthe plates 12, 14. The sealing material 16 surrounds the opening 24 onthe outer faces 30 of the plates 12, 14, preferably by way of beadedsections 28 located over grooves 42. The sealing material 16 alsosurrounds the opening 24 in the inside face 30 of one, or optionallyboth, plates 12, 14. The internal sealing material 16 required tosurround the opening 24 flows inwards from the edge 32 of a plate 12, 14through grooves 42 or gates 46 formed in the inside face 30 of the plate12, 14.

Alternatively or additionally, sealing material 16 required to surroundthe opening 24 may also be provided through one or more gates 46 throughthe thickness of a plate 12, 14. Sealing material 16 may be providedaround an opening 24 in the sub-assemblies 10 of the FIG. 1, 2, 4 or 5in a similar manner.

In further alternative structures, a coolant flow field may be providedin a separate plate rather than as part of the cathode plate 14 or anodeplate 12. The coolant field plate may be connected to an opening 24 inthe plate or to an external coolant jacket or to the atmosphere. In thiscase, some of the sub-assemblies 10 in a stack may be made as describedabove but without a coolant field 18 by omitting the coolant fieldplate. In sub-assemblies 10 with a coolant field 18, the coolant fieldplate is placed between the cathode plate 14 and anode plate 12 in amold and sealing material 16 is injected around the edges 32 of allthree plates as described above. The coolant field plate can be sealedto either, or both, of the cathode plate 14 and anode plate 12 by theedge sealing portion 26 alone or as shown for seals between the cathodeplate 14 and anode plate 12 in any of FIGS. 3 to 6.

The sealing material 16 both seals to the MEA when compressed in a stackand separate adjacent sub-assemblies 10 in a stack. Although manyindividual sub-assemblies must be made, the bead portions 28 assist inlocating the GDLs and MEAs while forming a stack. The various methodsdescribed in U.S. Pat. No. 7,210,220 to avoid shorting the fuel cellswhen using a seal in place are not required. The stack may bedisassembled and the MEAs examined if the stack is defective. Yet, theedges of the plates 12, 14 are sealed against coolant leakage withoutrequiring additional steps. In this way, the sub-assemblies 10 at leastprovide a useful alternative to the seal in place method. As discussedabove, in some cases gates through the thickness of the plates 12, 14can be reduced or eliminated.

Although the sub-assembly 10 has been described above for use in a PEMfuel cell, a sub-assembly 10 as described above may also be used inanother type of fuel cells, in a PEM or other type of electrolyser, orin electrolytic cells generally. The sub-assembly 10, and the method ofmaking it, may also be modified in various ways within the scope of theinvention, which is defined by the following claims.

1. A sub-assembly for an electrochemical stack comprising, a) a bipolarplate having upper and lower faces and an edge between the upper andlower faces, the upper and lower faces both containing flow fields; and,b) sealing material bonded to the bipolar plate and extending from theupper face of the bipolar plate, around the edge of the bipolar plate,and onto the lower face of the plate.
 2. The sub-assembly of claim 1wherein the sealing material also forms a bead around the periphery ofone or both of the flow fields.
 3. The sub-assembly of claim 1 whereinthe sealing material also forms one or more beads around one or moreholes for reactant, combustion product, or coolant flow through thebipolar plate.
 4. The sub-assembly of claim 1 wherein the bipolar platecomprises an anode plate and a cathode plate bonded together and acoolant flow field between the anode plate and the cathode plate.
 5. Thesub-assembly of claim 4 wherein the anode plate and the cathode plateare bonded together by sealing material which also provides a sealaround the coolant flow field.
 6. The sub-assembly of claim 4 whereinone or both of the plates has a gate or groove extending inwards fromits edge along an inner face of the plate.
 7. The sub-assembly of claim4 having a step or key in the edge of the bipolar plate.
 8. Thesub-assembly of claim 1 wherein the sealing material is one contiguousmass.
 9. A method of making a sub-assembly comprising the steps of, a)loading a single anode plate and a single cathode plate into a mold suchthat reactant flow fields on the plates face away from each other; b)injecting a liquid sealing material, for example liquid silicone rubber,into the mold, wherein the liquid sealing material fills a gap betweenthe edge of the plates, and portions of the outer faces of the plates,and the mold.
 10. The method of claim 9 wherein the liquid sealingmaterial also flows into one or more grooves on a coolant flow field onan inside face of at least one of the plates.
 11. The method of claim 9wherein the liquid sealing material also flows through one or moregrooves or gates extending inwards from the edge of at least one of theplates.
 12. An electrochemical stack having a plurality ofsub-assemblies as described above, or sub-assemblies made by the methoddescribed above.
 13. The stack of claim 12 further comprising a membraneelectrode assembly at least partially overlaps with the sealing materialon the upper face of the lower sub-assembly and with the sealingmaterial on a lower face of an upper sub-assembly.
 14. The stack ofclaim 13 further comprising a gas diffusion layer located between theupper face of the lower sub-assembly and the membrane electrode assemblyand within the sealing material on the upper face of the lowersub-assembly.
 15. The stack of claim 14 further comprising a second gasdiffusion layer located between the lower face of the upper sub-assemblyand the membrane electrode assembly and within the sealing material onthe lower face of the upper sub-assembly.